It is known to add different kinds of variable gain amplifiers (VGA) in front of the voltage to current converter.
Patent publication GB 2 334 163 shows a variable transconductance amplifier comprising two transistors whose emitters are connected via resistors to a constant current source to form a differential or long tail pair. A feedback amplifier whose transconductance is controllable has inputs connected to the collectors of the transistors and outputs connected to the emitters thereof. The feedback amplifier thus supplies a differential current to the emitters of the transistors, which corresponds to the product of the differential output signal of the transistors and the variable transconductance of the feedback amplifier. The arrangement is said to improve the noise figure.
Patent publication US 2003/0062951 shows a mixer/amplifier capable of providing variable gain while maintaining a substantially constant common mode operating voltage level and maintaining a substantially constant operating voltage in new, low voltage designs, thus providing a mixer/amplifier in low-power direct conversion receiver.
A low noise amplifier is shown in which a gain control signal is provided through a differential current source. Two output currents I1 and I2 are provided. I1+I2=a constant. I1 and I2 are mirrored in first and second paths, one including amplifier transistors and optionally a Gilbert cell multiplier. Irrespective of amplifier gain, the current through the two paths remains equal to I1 and I2, and common mode voltage remains constant.